Author:

D.P. Brennan(University of Tulsa)

The stability of multiple coupled resistive modes is examined
using reconstructions of experimental equilibria in the DIII-D
tokamak, revealing the important physics in mode onset as
discharges evolve to instability. Experimental attempts to access
the highest beta in tokamak discharges, including hybrid
discharges, are typically terminated by the growth of a large 2/1
tearing mode. In hybrid discharges the plasma current is
significantly noninductive, intended to lengthen the discharge
time, while sustaining the baseline dimensionless parameters of a
burning plasma experiment. Model equilibria, based on
experimental reconstructions from these discharges, are generated
varying $q_{min}$ and pressure. For each equilibrium the PEST3
code is used to determine the ideal MHD solution including both
tearing and interchange parities at all resonant surfaces. This
outer region solution must be matched to the resistive inner
layer solutions at each rational surface to determine resistive
mode stability. From this analysis we find that the approach to
$q=1$ resonance simultaneously causes the 2/1 mode to become
unstable and the nonresonant 1/1 displacement to become large, as
the ideal beta limit rapidly decreases toward the experimental
value. Here the 2/1 mode grows to a large size, leading to loss
of confinement. However, this nonresonant 1/1 component is
strongly coupled to the 2/2 harmonic of the unstable 3/2 mode,
which is thought to contribute to the current drive sustaining
$q_{min}$ above 1 in hybrid discharges. Thus, the approach to
$q=1$ resonance is self-limiting. Nonlinear coupling to the n=2
mode is computationally investigated using the NIMROD code. This
work suggests that sustaining qmin slightly above 1 will help
avoid the 2/1 and allow access to significantly higher beta
values in these discharges.

*Supported by US DOE under DE-FG03-95ER54309 and DE-FC03-04ER54698.

To cite this abstract, use the following reference: http://meetings.aps.org/link/BAPS.2006.DPP.VI2.3